There have been many advances in artificial lighting since humans first began wanting to see after the sun went down. Torches have been replaced by candles and candles by oil lamps, gas lamps and eventually electric lights. There are many types of illumination, including incandescent lamps, arc lamps, metal vapour lamps, discharge tubes, various types of fluorescent lamps and more recently light emitting diode (LED)-based lamps. While there are many ways to create light, human colour vision evolved using the light from the sun and it is this to which all other lighting systems are compared. An important feature of human vision is the ability to identify many different colours. Colour vision results from a complex interaction between illumination, the properties of the object being viewed, the physiology of the eye, and the processing of visual stimulae by the human brain.
In order to reliably perceive the colour of an object, it is important that the object be illuminated with wavelengths of light that closely mimic the light of the sun. Some artificial light sources such as arc lamps do an excellent job of this, while others such as certain types of fluorescent lamps or LED-based lamps do not. This variability in the quality of colour rendering of illumination has been quantified, in a measure known as the colour rendering index (CRI) of a light source. After brightness of illumination this is one of the most important characteristics of a light source, particularly light sources designed to illuminate commercial facilities such as offices, manufacturing facilities and retail facilities such as stores or shopping malls. Light sources with high quality colour have a colour rendering index approaching 100, the value for natural lighting by the sun.
LED based lamps have become popular due to their energy efficiency and long lifetime, and are increasingly being incorporated in luminaires for office and home use. A luminaire is an enclosure that provides a way of mounting a lamp and can incorporate electrical connectors, power conditioners or converters, light directing elements such as reflectors and light diffusing elements such as the moulded plastic panels often seen in fluorescent lamp luminaires in an office. While LED based luminaires have advantages there are also disadvantages. LED luminaires are more costly to make, but do not require regular replacement of bulbs or fluorescent tubes because they lose much less of their brightness over time. Older type luminaires required frequent lighting element replacement and so changes in colour illumination properties over time were also easily rectified. This is not the case with LED luminaires, since the LEDs are permanently mounted on electronic circuit boards and generally require replacement of the entire luminaire or at least a major component.
Another problem with LED luminaires is that while they may be bright and efficient, the colour rendering index may not be suitable for all types of uses. For example, an industrial building used for light manufacturing, may be subsequently leased by fashion design house requiring higher quality illumination. The lighting may need to be adjusted to provide an improved colour rendering index.
Another problem facing LED luminaires is that they are composed of multiple types of LEDs and over long periods of time the colour properties of the different LEDs may change relative to one another, resulting in changes in colour rendering index. Another issue of luminaire aging is that the life times of the different types of LEDs may vary. For example, over a period of years 3% of LEDs of one colour in the luminaire might fail, while 6% of LEDs of another colour in the luminaire might fail, producing a gradual change in colour properties.
Another issue of LED luminaires results from slight variations between luminaires when newer luminaires or luminaires from other manufacturers are added, to an office for example to increase brightness or replace failed or damaged systems, but then do not quite match in colour. This can produce an undesirable aesthetic effect due to perception of the luminaire colour mismatch.
It is an object of the present disclosure to provide an active diffuser that may be incorporated in a luminaire, added to a luminaire, or that can replace an existing diffuser of a luminaire, to controllably improve the colour rendering characteristics of the illumination light, compensate for effects due uneven aging of LEDs and that can correct for appearance mismatch between luminaires.